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How to configure medium voltage switchgear: Key influences and stress variables

Home / Technical Articles / How to configure medium voltage switchgear: Key influences and stress variables

Medium Voltage Switchgear

Let’s discuss the most critical factors that influence the correct configuration of medium-voltage switchgear. As you will see below, for the most common applications of MV switchgear, there are at least six main categories that are very important in the design of future power distribution.

Configuration of medium voltage switchgear: The most critical factors
Configuration of medium voltage switchgear: The most critical factors

As you will see, these categories are MV network characteristics, protection, measurement and metering concepts, the operating principle and number of infeeds, installation location and accessibility, environmental conditions, and requirements for operating procedures.

The most essential industry-specific application requirements for medium-voltage switchgear, such as switching-duty-capacity, switching frequency of the loads, switchgear modifications or extensions, are also covered.

The key influences and stress variables for switchgear depend on the task in hand and its position in the distribution network. These influencing factors and stresses, which are explained in detail in this article, determine the selection and rating variables for the switchgear.

Take your coffee, and let’s get on!


Table of Contents:

  1. MV network characteristic values:
    1. Line voltage
    2. Short-circuit current
    3. Normal current and load flow
    4. Neutral point earthing
    5. Underground / overhead lines
    6. Overvoltage protection
    7. Power quality (unstable loads)
  2. Line Protection, measurement and metering concepts:
    1. Short-circuit protection
    2. Relay protection functions
    3. Tripping times
    4. Measurement and metering
    5. Redundancy
  3. Infeed types
  4. Operating sites:
    1. Installation location
    2. Accessibility
    3. Switchgear room
    4. Buildings
    5. Transportation and assembly
  5. Environmental conditions:
    1. Ambient room conditions
    2. Altitudes above 1000 m
    3. Ambient temperature and humidity
  6. Industry-specific application:
    1. Switching duty and capacity
    2. Switching frequency of the loads
    3. Frequency of switchover between busbars
    4. Availability (faults, redundancy, switchover time)
    5. Switchgear modifications or extensions
  7. Operating procedures:
    1. Operational activities
    2. Work (maintenance) activities
    3. Inspection and maintenance
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1. MV network characteristic values

1.1 Line voltage

This determines the rated voltage of the switchgear, switching devices, and other installed components. The determining factor here is the maximum line voltage at the upper tolerance limit.

Assigned configuration criteria for switchgear:

  • Rated voltage Ur (or Us)
  • Rated insulation level Ud (or Upn)
  • Rated primary voltage of voltage transformers Upr and surge arrester – where applicable

Table  1 – Rated voltage of the primary circuit – General case

Rated voltage of the primary circuit - General case
Rated voltage of the primary circuit – General case

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1.2 Short-circuit current

This is characterized by the variables peak current Ip (peak value of the initial symmetrical short-circuit current) and sustained short-circuit current Ik. The required short-circuit current level in the network is specified by the dynamic behaviour of the loads and the quality of the energy, which must be maintained, and determines the making, breaking, and withstand capacity of the switchgear.

Note that the ratio of the peak current to the sustained short-circuit current in the network can be significantly greater than the standard factor Ip/Ik = 2.5 at 50 Hz (or 2.6 at 60 Hz) according to which the circuit-breakers and switchgear are designed.

This could be due, for example, to motors that supply energy back to the network if a short-circuit occurs, which significantly increases the peak current. For the short-circuit duration, see Section 2.3 Tripping times

Assigned configuration criteria for switchgear:

  • Main and earthing circuit:
    • Rated peak current Ip
    • Rated short-time current Ik
  • Switchgear:
    • Rated short-circuit making current Ima
    • Rated short-circuit breaking current Isc
  • Current transformer:
    • Rated dynamic current Idyn
    • Rated short-time thermal current Ith

Figure 1 – An example of short circuit in 10kV medium-voltage switchgear

What do you need for short-circuit calculations? The less you assume, the better!
An example of short circuit in 10kV medium-voltage switchgear

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1.3 Normal current and load flow

The normal current refers to the current paths of the infeeds, busbar(s), and load feeders. The current is also subdivided due to the physical arrangement of the switchgear panels with the effect that different rated current values can be found in sequence along the conducting path (the currents in busbars and feeders are normally different).

Reserves must be planned for the value of the rated normal current:

  1. In accordance with the ambient temperature;
  2. For the planned overload;
  3. For temporary overloads in the event of faults.
With high normal currents, large cable cross-sections or several parallel cables must be connected in the switchgear panel; the field terminal must be designed to accommodate this.

Assigned configuration criteria for switchgear:

  • Rated current of busbar(s) and feeders
  • Number of cables for each conductor in the switchgear panel (parallel cables)
  • Rating of the current transformers

Figure 2 – Initial Load Flow Analysis in ETAP

Initial Load Flow Analysis
Figure 2 – Initial Load Flow Analysis in ETAP

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1.4 Neutral point earthing

The type of neutral point earthing used, insulated / resonant earthed with an arc suppression coil (Petersen coil) / (temporarily) resistively earthed / solidly (directly) earthed – governs the behaviour of the line voltage when switching operations are carried out or if a earth fault occurs in the network as well as the characteristics of the earth fault currents.

This means that neutral point earthing has a major influence on the dielectric stress on the equipment and is important for insulation coordination.

Assigned configuration criteria for switchgear:

  • Insulation level of the equipment assigned to the rated voltage Ur
  • Choice of switchgear
  • Rating and damping of voltage transformers
  • Design of current transformers and protection relays for earth fault detection
  • Rating of surge arresters

Figure 3 – MV Neutral Earthing Methods and Their Applications

MV Neutral Earthing Methods and Their Applications throughout the World
Figure 3 – MV Neutral Earthing Methods and Their Applications throughout the World

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1.5 Underground / overhead lines

The network type governs insulation coordination and which overvoltage protection measures need to be taken. In overhead systems, powerful overvoltages can be caused by a lightning strike, while only weak switching overvoltages generally occur in underground networks. Overvoltages from overhead systems can reach switchgear indirectly or via a transformer.

These connecting points must be taken into account and checked to determine whether surge arresters are needed.

The network type also affects the choice of switchgear: in overhead systems, for example, transient faults are more likely to occur. Circuit-breakers must be able to manage reclosing cycles and a high number of make-break operations. The short-circuit currents in medium-voltage overhead systems, however, are low (generally between 8 kA and 12 kA).

Assigned configuration criteria for switchgear:

  • Selection and rating of circuit-breakers
  • Insulation level
  • Use and rating of surge arresters

Figure 4 – Transient stability analysis enables engineers to accurately simulate and analyze power system dynamics and transients via system disturbances and other events

Transient stability analysis enables engineers to accurately simulate and analyze power system dynamics and transients via system disturbances and other events
Figure 4 – Transient stability analysis enables engineers to accurately simulate and analyze power system dynamics and transients via system disturbances and other events

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1.6 Overvoltage protection

If the expected voltage stresses exceed the insulation level of the equipment, insulation coordination requires overvoltage protection. This can apply, for example:

  • When external overvoltages can be expected as a result of lightning strikes;
  • When (multiple) earth faults occur frequently;
  • High transient overvoltages may occur as a result of switching operations;
  • When, in the interests of economy, certain equipment (e.g. motors, resin-encapsulated / dry-type transformers) is rated for lower insulation levels than the other network components.

Overvoltage protection is also used as a general precaution to minimize the risk of failure and to protect equipment against any form of overvoltage. It is generally used for very large motors or transformers that are not only expensive and difficult to procure but also incur high costs if they fail.

Frequent overvoltages can also cause some insulation systems to age, even if the stresses are still within permissible limits. Overvoltage protection measures are also recommended in “unstable” networks (see the Section 1.7 Power quality).

Overvoltage protection systems normally comprise surge arresters. In certain cases, surge capacitors or resistance-damped surge capacitors (RC elements) are also required. Switchgear must be checked to determine whether overvoltage protection is required, whether protection measures have already been taken, or whether existing protection devices need to be upgraded.

Assigned configuration criteria for switchgear:

  • Insulation level
  • Check for “critical” switching duties
  • Application and rating of the surge arresters, space for installtion and possibility for the connection of external arresters

Further Study – MV/LV transformer protection against temperature rise, overloads, short-circuit & overvoltages

MV/LV transformer protection against temperature rise, overloads, short-circuit & overvoltages

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1.7 Power quality (unstable loads)

The power quality refers to unwanted interference, such as voltage dips, flickers, asymmetry, harmonics etc.. These can be caused for example by rectifiers, converters, welding machines, direct-starting motors. Other loads (e.g. IT systems), however, are sensitive. To protect them, the interference is compensated as much as possible or the “stable” and “unstable” loads are distributed across sub-networks with separate infeeds.

Depending on the circuit arrangement, switchgear with single or double busbars is required.

Assigned configuration criteria for switchgear:

  • Single/double busbar:
    • Bus section panel
    • Bus coupler unit
  • Rated normal current (busbar)
  • Rated short-circuit currents

Figure 5 – Two interconnected double busbars

Figure 5 – Two interconnected double busbars

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2. Line Protection, measurement and metering concepts

2.1 Short-circuit protection

The line protection concept has a major influence on the switchgear that is selected and its design. The following devices can be used for short-circuit protection:

  • HRC fuse (in a switch- or contactor-fuse combination)
  • Instrument transformer + protection relay + circuit-breaker
If the majority of network branches are equipped with switch-fuse combinations, switch-disconnector units can be used; otherwise, circuit-breakers are used.

Assigned configuration criteria for switchgear:

  • Design: circuit-breaker or switch-disconnector unit

Figure 6 – Racking-in and racking-out operation of MV circuit breaker

Racking-in and racking-out operation of MV circuit breaker
Figure 6 – Racking-in and racking-out operation of MV circuit breaker

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2.2 Relay protection functions

Protection relays obtain their measurement signals from current and voltage transformers, which are installed in the switchgear. The range of available protection functions (e.g. inverse/definite-time overcurrent, distance/differential protection) requires current transformers whose cores can be combined in a variety of ways with different rated currents, overcurrent number, output, and class accuracy.

Voltage transformers are also provided for protection with directional control or for determining the fault location (distance protection).

Sensitive earth fault protection devices require a core-balance transformer around the three conductors to measure small earth fault currents in compensated networks or networks with an insulated neutral point.

The switchgear must provide a space in which these instrument transformers can be installed as well as space for the protection relays and their wiring. This may seem obvious, but it is important to remember that other components (surge arresters, multiple cable terminals etc.) also need space in the switchgear.

Figure 7 – CTs and VTs compartment in MV swtichgear

Choosing CTs for differential protection applications (motors, generators, busbars and REF)
CTs and VTs compartment in MV swtichgear – photo credit: Edvard Csanyi

Assigned configuration criteria for switchgear:

  • Installation of instrument transformers in the switchgear panel and on the busbar
  • Installation of a core-balance transformer (normally cable-type current transformer, or “earth fault winding”)
  • Installation of protection relays and wiring in the low-voltage compartment

Further Study – Protection Relay – ANSI Standards

Protection Relay – ANSI Standards

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2.3 Tripping times

The set tripping times (specified by means of selectivity requirements in accordance with the number of subordinate network levels and their equipment) are governed by the rated short-circuit duration of the switchgear, the current transformer, and the earthing circuit. The standards permit various rated values for the components.

For short-circuit durations of > 1 s in particular, it is important to ensure that all components are dimensioned to manage the actual short-circuit duration at the minimum.

For the actual short-circuit duration, the mechanical delay (opening time) of the switching device must also be added to the tripping time of the protection system.

Assigned configuration criteria for switchgear:

Further Study – The essentials of necessary auxiliary relays in tripping and control applications

The essentials of necessary auxiliary relays in tripping and control applications

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2.4 Measurement and metering

Measurement and metering systems generally require separate, additional transformer cores with a different rating to that of the protection cores.

Assigned configuration criteria for switchgear:

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2.5 Redundancy

Different protection functions can be combined in a single network branch (e.g. differential and overcurrent-time protection as backup). Depending on the degree of redundancy required, additional instrument transformers and protection devices may also have to be installed in the switchgear. Enough space must be available for these too (see above).

For contractual reasons, the same measurement device or meter may also be installed twice if contractual partners use their own devices at the point of supply (“check measurement”). This increases the number of built-in components.

Assigned configuration criteria for switchgear:

Figure 8 – Low-voltage compartment with auxiliary relays in relay protection unit in MV switchgear

The essentials of auxiliary relays in relay protection needed for analyzing schematics
Figure 8 – Low-voltage compartment with auxiliary relays in relay protection unit in MV switchgear

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3. Infeed types

The operating principle and number of infeeds are crucial for connecting the switchgear and for its rated current values. The supply from different networks, such as:

  • the public network,
  • in-plant generation, and
  • the emergency power supply

… sometimes has to be disconnected during normal operation for safety, business, or contractual reasons. A decision must, therefore, be made regarding how the supply can be divided into busbar sections and which ones can be linked.

The switchgear design as well as the rated normal and short-circuit current values depend on the infeed operating principle and the couplings.

Assigned configuration criteria for switchgear:

  • System connection:
  • Rated values of the switchgear:
    • Normal current of the busbar(s)
    • Peak current and short-time current
  • Control, interlocks, and switchgear interlocking
  • Installation of instrument transformers in the switchgear panel and on the busbar
  • Room for installing the protection relays and wiring in the low-voltage compartment

Watch Video – Introduction to incoming feeder

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4. Operating sites

The type of operating site can also govern the choice and rating of switchgear. Given the broad range of different influencing factors, only the key points that need to be taken into account can be described here.


4.1 Installation location

Many locations are subject to legal regulations concerning safety and health, fire protection, and environmental compatibility. This mainly applies to systems in public areas (e.g. pedestrian zones), industrial premises (offices, workshops), public buildings (high-rises, hospitals, office buildings, bars, conference centers etc.).

Special regulations also apply for nature reserves, mines, railways, and boats.

Even if the switchgear malfunctions, it must behave in such a way as to minimize negative impact. Malfunctions include situations in which the switchgear is subject to external damage (e.g. fire) or is itself the cause of the problem (e.g. internal faults).

Assigned configuration criteria for switchgear:

  • Design
  • Internal arc fault classification
  • Pressure absorber, pressure release duct

Figure 9 – Indoor metal-clad medium voltage switchgear

Indoor metal-clad medium voltage switchgear
Figure 9 – Indoor metal-clad medium voltage switchgear

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4.2 Accessibility of MV switchgear

The standards also take into account the different levels of accessibility of operating sites:

  1. Only authorized and trained personnel have access to closed electrical operating area; all other people must be accompanied.
  2. Operating sites in public areas can be accessed by everyone (e.g. standard for stations and switchgear in workshops).
Switchgear in public areas are subject to more stringent requirements.

Assigned configuration criteria for switchgear:

  • Air or gas insulation
  • IP degree of protection
  • IK degree of protection (mechanical shock)
  • Internal arc fault classification

Figure 10 – Typical incomer feeder cubicle (front, side and rear view)

Incomer feeder cubicle (front, side and rear view)
Figure 10 – Typical incomer feeder cubicle (front, side and rear view)

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4.3 Switchgear room

Certain standards, e.g. IEC 61936, and laws define specifications regarding how the system is set up in service rooms (e.g. the minimum width of operating and assembly aisles) and define the binding requirements for escape routes (e.g. width and maximum length of escape routes, preferred direction in which doors close etc.).

The service room might also contain other equipment for which operators can define their own specifications regarding free areas and setup. The switchgear must enable all requirements to be fulfilled.

Assigned configuration criteria for switchgear:

  • Dimensions of the switchgear panels, in particular the panel width
  • Position of door stops; opening angle of the switchgear panel doors
  • Position of controls and displays/indicators (front/rear)
  • Cable connection from the front or rear
  • Required accessories (switching levers, tools, etc.)
  • Labelling, information plates, warning signs

Figure 11 – MV switchgear room

MV switchgear room
Figure 11 – MV switchgear room

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4.4 Buildings

The building itself can also influence the choice of switchgear. The following aspects must be taken into account:

  1. The available space
  2. The quality of the building fabric (in existing, older buildings)
  3. Vent outlets in the event of an internal arc fault

Assigned configuration criteria for switchgear:

  • Switchgear panel width, depth, and height
  • Air or gas insulation
  • Internal arc fault classification
  • Pressure absorber, pressure release duct
Irrespective of the chosen switchgear, a dynamic pressure calculation must be carried out for the building to determine the stresses on the structure and the required cross-sections for vent outlets. The building is also a crucial factor regarding the conditions under which the switchgear operates.

See Section 5. Environmental conditions.

Figure 12 – Switchgear ventilation: a) Simple compartment ventilation, b) compartment ventilation with exhaust hood above the switchboard, c) ventilation with false floor, d) ventilation with recirculating cooling system

Compartment ventilation
Figure 12 – Switchgear ventilation: a) Simple compartment ventilation, b) compartment ventilation with exhaust hood above the switchboard, c) ventilation with false floor, d) ventilation with recirculating cooling system

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4.5 Transportation and assembly

In certain – albeit rare – cases, transportation and assembly conditions are a key selection criterion for the switchgear. Whether or not switchgear can be installed in a building depends on certain constructional factors, such as:

  1. The size and position of doorways
  2. Permissible floor loading
  3. Floor level
  4. Goods elevator

Assigned configuration criteria for switchgear:

  • Size of the transportation units
  • Weight of the transportation units
  • Packaging (stability, weather protection if the device is in temporary storage over a long period)

Figure 13 – Transport of low voltage and medium voltage switchgear on site

Transport of low voltage switchgear on site
Figure 13 – Transport of low voltage and medium voltage switchgear on site

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5. Environmental conditions

5.1 Ambient room conditions

Switchgear installed in service rooms can be subject to the following ambient conditions:

  1. Heat and humidity
  2. Pollution
  3. Dust and smoke
  4. Salt (near the coast or in mining)
  5. Corrosive gases and vapours (natural and artificial)
  6. Insects / small animals

Due to the infinite variety of ambient conditions at installation locations, the applicable standards only define basic requirements for “normal service conditions”. Each system much be examined individually to determine whether or not these are observed.

Numerous measures can be taken to provide protection against exceptional climatic conditions. For example, gas-insulated, hermetically sealed compartments that are completely immune to external conditions can be used. The degree of protection of the housing can also be increased. In parts of the world where high insect/small animal populations can cause problems, protection against bridging may be appropriate by fitting additional insulation to live and exposed components.

Assigned configuration criteria for switchgear:

  • Design: air or gas-insulated switchgear
  • IP degree of protection for housing (can be increased with drip-water protection if necessary)
  • Type of ventilation
  • Additional insulation for live and exposed components

Measures to provide protection against extreme ambient conditions can also be taken in the service room (air conditioning). In some cases, this can be more efficient than designing each individual component accordingly.

It is also important to remember that protection, measurement, and control systems are more sensitive than switchgear, which is reason enough to ensure that the minimum requirements for air quality are observed.

Figure 14 – Partial disharge on MV busbars caused by air pollution

Partial disharge on busbars
Figure 14 – Partial disharge on MV busbars caused by air pollution

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5.2 Altitudes above 1000 m

The standardized insulation levels apply to normal ambient conditions at sea level. As the altitude increases, however, the air density decreases as do the insulating properties. The applicable standards take this into account and allow for a reduction of approx. 9% at altitudes of up to 1000 m.

At higher altitudes, the required insulation level must be ensured by increasing the rated voltage (with respect to sea level values).

Insulation coordination can also be ensured by means of additional surge arresters. Alternatively, gas-insulated switchgear in which the primary section is hermetically sealed can be used. In this case, the external air pressure is not an issue.

Assigned configuration criteria for switchgear:

  • Design: air or gas-insulated switchgear
  • Selection of a higher rated voltage with a higher insulation level
  • Use of surge arresters

Suggested Course – Surge Protection Devices (SPDs) Course: Design, Specification and Installation For True Engineers

Suggested Course - Surge Protection Devices (SPDs) Course: Design, Specification and Installation For True Engineers
Suggested Course – Surge Protection Devices (SPDs) Course: Design, Specification and Installation For True Engineers

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5.3 Ambient temperature and humidity

The ambient temperature directly affects the temperature of the switchgear which, in turn, affects its current-carrying capacity. As the ambient temperature increases, the current-carrying capacity decreases (and vice versa). To compensate for this, either the rated current value must be increased or sufficient ventilation must be ensured (ventilation openings affect the IP degree of protection).

Humidity has a negative effect on the insulating properties. High levels of humidity together with sudden temperature changes can cause condensation which, in turn, drastically reduces insulation levels. To prevent this, controlled or permanent heaters can be used in conjunction with humidistats.

Assigned configuration criteria for switchgear:

  • Rated normal current
  • Type of ventilation
  • Differentiation of IP degree of protection according to protection against ingress of solid foreign bodies and shock-hazard protection
  • Installation of a heater in the switchgear panel or in the low-voltage compartment

Figure 15 – Ambient temperature (bottom curve) and on B Phase Top FC (top curve) in the same cell without ventilation (A) and with forced ventilation (B)

Ambient temperature (bottom curve) and on B Phase Top FC (top curve) in the same cell without ventilation (A) and with forced ventilation (B)
Figure 15 – Ambient temperature (bottom curve) and on B Phase Top FC (top curve) in the same cell without ventilation (A) and with forced ventilation (B)

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6. Industry-specific application

The following aspects cover the role and significance of the switchgear in the operator’s network (industrial or public utility companies).


6.1 Switching duty and capacity

The choice and rating of the switchgear depends on the switching duty and the loads to be switched. Is the switchgear only required to switch normal currents? Or does it need to interrupt short-circuits too? Due to the transient switching operations, certain loads require overvoltage protection.

In other cases, additional rating criteria apply to the switch and switchgear.

Assigned configuration criteria for switchgear:

Further Study – MV/HV switchgear (circuit breaker) switching capability and suitability for specific applications

MV/HV switchgear (circuit breaker) switching capability and suitability for specific applications

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6.2 Switching frequency of the loads

Alongside the electrical requirements regarding the switching capacity, the switching frequency is an important selection criterion. The switching frequency depends on the process in which the switching device is used. Switches are rated for a short electrical lifetime (number of make-break operations), while contactors are rated for extremely long lifetimes; circuit-breakers fall in between.

Once a basic decision has been reached regarding the type of switchgear, standardized lifetime increments can be selected for the switches (endurance classes).

For higher switching frequencies under normal operating conditions, switchgear must be serviced or replaced more often. In this case, the components must be easily accessible.

Assigned configuration criteria for switchgear:

  • Switching devices: circuit-breaker, switch or contactor
  • Classes for mechanical and electrical endurance
  • Design of the switchgear assembly
    • Air or gas-insulated system
    • Withdrawable or fixed disconnector
  • Loss of service continuity category

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6.3 Frequency of switchover between busbars

In switchgear with double busbars, the lifetime of the disconnector plays a crucial role. For operational reasons, some networks require frequent switchovers between busbars. Due to the short mechanical lifetime of a disconnector / withdrawable unit as compared with a circuit-breaker, “standard” double busbar switchgear systems with just one circuit-breaker are unsuitable for this type of operation.

Standard double busbar systems have a common connection (cross connection between the two busbars) between the two disconnectors and the circuit-breaker.

To prevent wear and tear to the disconnector / withdrawable unit and, in turn, minimize maintenance cycles, the busbars are switched by means of circuit-breakers. This requires two single busbar systems each with two circuit-breakers in one feeder circuit.

Assigned configuration criteria for switchgear:

  • Design of the switchgear
  • Double busbar with common connection or two single busbars
  • Interlocks and switchgear interlocking

Watch Video – Bus Section Interlocking

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6.4 Availability (faults, redundancy, switchover time)

The term “availability” in this context means providing ways of compensating for line failures, for example, by means of redundant supply channels or defined switchover times. It also refers to the reliability of switchgear and features for planned maintenance activities or troubleshooting.

The switchgear provides the current paths for multiple supply channels and the switchover options. This can be used to determine the system circuit: single or double busbars and separated into different sections. The various options for linking busbars (busbar sections) with separate infeeds and the operating principle (NO / NC) of the couplings govern the rated current values.

The switchgear itself must also be sufficiently reliable, maintenance friendly (and not require excessive maintenance), and accessible. A decision regarding the Design of the switchgear assembly and the partitions can be made on the basis of there criteria.

Assigned configuration criteria for switchgear:

  • Circuit arrangement:
    • Single/double busbar
    • Bus section panel
  • Interlocks and switchgear interlocking
  • Rated values of the switchgear:
    • Normal current of the busbar(s)
    • Peak current and short-time current
  • Design: air or gas-insulated switchgear
  • Loss of service continuity category:
    • Design of partitions

Figure 16 – Example for the service continuity (LSC) of medium voltage switchgear

Example for the service continuity (LSC) of medium voltage switchgear
Figure 16 – Example for the service continuity (LSC) of medium voltage switchgear

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6.5 Switchgear modifications or extensions

Switchgear is sometimes set up in sub-sections or needs to be extended or modified during the course of its operating life. If such measures are anticipated, these should be taken into account in the planning stage.

Assigned configuration criteria for switchgear:

  • Design
  • Extension to include additional switchgear panels
  • Rated values for normal and short-circuit current
  • Replacement of instrument transformers
  • Option of upgrading the secondary system

Further Study – Substation design choices and reasons for a new modern vs retrofit and upgrade an old one

Substation design choices and reasons for a new modern vs retrofit and upgrade an old one


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7. Operating procedures

Operating procedure is a catch-all term that, with respect to configuration activities, will only be used to describe the following activities:

  • Operation
  • Work activities
  • Maintenance

7.1 Operational activities

Operational activities include monitoring, switching, making settings, and reading displays / indicators. Operators can define how these activities are to be carried out, that is, whether they are to be carried out on site or remotely, (completely or partially) manually or automatically.

The automated integration of the switchgear in the network system management and production process makes important demands on the measurement, control, and remote control systems (secondary technology).

During operation, the switchgear must provide the required level of personnel protection (e.g. protection against accidental contact with hazardous live or mechanical components (IP degree of protection). Even in the (unlikely) event of an internal fault, the switchgear must offer means to protect the operator.

Assigned configuration criteria for switchgear:

  • Design of the switchgear
    • Air or gas-insulated system
    • Withdrawable or fixed disconnector
  • Type of compartments (access control)
  • Loss of service continuity category
  • Control and secondary equipment
  • Interlocks and switchgear interlocking
  • Manual or automatic drive mechanisms
  • IP degree of protection
  • Internal arc fault classification

Suggested Course – Learn How to Operate and Analyze Interlocking Schemes for Substation & Gas Insulated Switchgear (GIS)

Learn How to Operate and Analyze Interlocking Schemes for Substation & Gas Insulated Switchgear (GIS)
Learn How to Operate and Analyze Interlocking Schemes for Substation & Gas Insulated Switchgear (GIS)

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7.2 Work (maintenance) activities

Activities in this context refers, for example, to modification, servicing, and maintenance activities or replacing fuses. When such activities are carried out, the system must be either fully or partially taken out of service.

How much of the system is taken out of service depends on the ‘loss of service continuity category’, which must be defined by the operator along with the type of access control to the disconnected compartments.

Shock-hazard protection for live components can, to varying degrees, be installed permanently in the system or implemented manually. Again, the operator must decide which method to use. How the work procedures are organized and the level of training of the personnel play an important role here.

Assigned configuration criteria for switchgear:

  • Loss of service continuity category
  • Type of compartments (access control)
  • Partition class

Figure 17 – Maintenance and testing Of medium voltage switchgear

Operation, Maintenance and Testing Of Electrical Power Equipment (Theory and Practice)
Figure 17 – Maintenance and testing Of medium voltage switchgear (photo credit: TUYEN LAM TECHNOLOGY ELECTROMECHANICS CO., LTD)

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7.3 Inspection and maintenance

System inspection and maintenance requirements can vary enormously and are subject to numerous influencing factors completely unrelated to electrical engineering, such as:

  1. Environmental conditions
  2. Industry-specific operating procedure
  3. Legal regulations
  4. Internal organization and work instructions
  5. Level of personnel training

Inspection and maintenance must be treated as separate activities. It may be the case that certain procedures require more frequent inspections, for example, which means that the switchgear must be more readily accessible. In this case, therefore, a completely hermetically-sealed system may not be the best solution because it cannot be accessed.

Components may need to be readily accessible if, for instance, they need to be replaced more often due to a high switching frequency. The degree of required accessibility also depends on various regulations and the level of personnel training. These aspects also govern the switchgear design.

Assigned configuration criteria for switchgear:

  • Design of the switchgear
    • Air or gas-insulated system
    • Withdrawable or fixed disconnector
  • Loss of service continuity category
  • Type of compartments (access control)
  • Selection of switching devices and its endurance classes

Further Study – Prepare yourself for successful inspection and testing of MV metal-clad switchgear

Prepare yourself for successful inspection and testing of MV metal-clad switchgear

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8. BONUS (PDF): Protection Study for 132kV, 33kV and 11/6.6kV Primary Substations

Download: Protection Study for 132kV, 33kV and 11/6.6kV Primary Substations (for premium members only):

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References:

  1. Electrical Calculations and Guidelines for Generating Stations and Industrial Plants by T.E.Baker

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Edvard Csanyi - Author at EEP-Electrical Engineering Portal

Edvard Csanyi

Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.

I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.

Profile: Edvard Csanyi

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